This isn't a novelty in the psychiatric genetics world. The same holds true for schizophrenia, depression and other mood disorders, and most other brain disorders for that matter. It is likely that this has to do with an interaction between disease genes and environmental factors, other genetic factors, or with stochastic (random) processes.
It's not that different than most other complex diseases. For example, you may carry a risk allele for heart disease. If you follow the right diet, and have a blissful, stress-free life, you might be in luck. But, if you're carrying a second risk allele (whoops!), or down a few too many Big Macs... that risk allele will bite you in the ass. For that matter, even if you do take care of yourself, that risk allele may still bite you in the ass. It's an odds game, and each risk factor makes the odds that much worse.
As for your "experiment that everyone conveniently chooses to forget"-- there's an extensive literature of twins with Autism. It also shows that there is a strong genetic component, but it's not absolute. The concordance of Autism in twins is extremely high -- but not absolute. However, even identical twins have significant differences -- yes, even genetically. And, even though they may share the same womb, they may have siginficant differences in fetal nutrition (depending on how the placenta is located), and they may be subject to different gestational stresses or birth trauma.
In other words -- nobody's conveniently forgetting anything.
Or, to put it more charitably, medicine and psychology are far describing far more complex phenomenon than we like to admit.
For example, in psychiatric genetics, there are dozens of articles every year that find a new gene associated with a common and important condition (e.g. autism, schizophrenia, depression). After each new finding comes out, there are dozens of labs that try to replicate that finding, usually one or two replicate (or partially replicate) the finding, and five or six don't replicate it. Why is it so hard to replicate these findings? Probably because there are really dozens of independent genes that contribute to these complex disorders (probably in combination with each other), and some populations tend to have mutations in one set, while other populations tend to have mutations in another set.
We're moving towards understanding, but the disorders are far more complex than the assumption that there will be a single cause.
Also, there's not that much literature on RGS14 at this point (it doesn't seem to have come up in any of the GWAS -- wide scale genome association studies) for psychiatric disorders, but it has been identified in molecular studies as a target of P53 (a central cancer regulatory mechanism). It would not be out of the question for this knockout to have a significant increase in cancer risk (brain or elsewhere), but not have this detected in a small-scale study.